The present invention relates to computer input devices such as graphics tablets, touch pads, and touch screens.
Input devices for sensing the position of an object such as a finger tip or stylus tip in two dimension are in wide use as computer input devices. The uses of such devices include tablets and touch screens mounted in front of cathode ray tube or liquid crystal displays. A common construction for such tablets for displays uses two transparent members each with a uniform thin coating of a resistive material such as indium tin oxide (ITO). The coated members are held separated by a small distance. When a stylus or finger tip is pressed against one of the members, the two conductive surfaces are brought into contact forming a resistive voltage divider. Since the surfaces have a uniform resistance, by driving a potential difference across one member and measuring the voltage picked up on the other member, a voltage proportional to the position of the contact point in the driven member is transferred to the sense member. Reading this voltage gives the stylus position. Reading the position for the other axis involves switching the driven and sensing members. In this manner, the X-Y position of the stylus or finger tip can be read. By repeating readings, the position of the stylus or finger tip can be tracked.
Reading the position of a stylus or finger tip by reading the tablet voltage is commonly performed in the art by a microprocessor controlling an analog to digital converter. With a 12 bit analog to digital converter, the tablet surface is effectively divided into a grid of 4096 by 4096 points. The microprocessor typically makes hundreds of readings per second to track the position of the stylus or finger tip.
A number of problems exist with this scheme. Stable position readings require a single, stable contact point of a minimum size and a minimum pressure defined by the characteristics of the tablet. If the stylus or finger tip pressure on the tablet is too light, the contact point will not be stable, the sensed voltage will vary, and erratic readings will result. Another very common problem arises when a user unfamiliar with the device rests the palm of his or her hand on the tablet while writing with the stylus, presses on the tablet with multiple fingers at once, or presses on the tablet with a large object such as the thumb or heel of the hand. Large and multiple contacts result in varying voltages, causing erratic readings. Electrical noise generated in surrounding circuitry such as the display, electrical noise picked up from the surrounding environment, and noise generated by current flow in the resistive layers of the tablet also produce small variations in voltage, called jitter, that can result in erratic readings.
Because the computer system is making many tablet readings per second, these erratic readings may be interpreted to indicate very rapid movement, producing unintended results. What is needed is a simple way to scan a graphics tablet to reduce the effects of jitter and erratic readings, and to detect light pressure, multiple objects, and large objects on the tablet so that the computer system may take appropriate action.